Have had a similar problem with the G/D pickup on a Peavey Fury. Checked wiring etc and all was good
Note that some types of strings are less magnetic than others. Check and note string heights above pole-pieces for reference before adjustment of pickups. If raising the pickup yields minimal results, try magnets.
You CAN re-magnetize pickups...
I used 4 of those super strong toy magnets that my kids have in a kit, they build shapes with them. They were neodymium.
I stuck one per pole on both pickups (4 poles) and let them sit on the bass for a week. Output was much more even across the poles (strings) after removal.
Then out of curiosity, I applied a smaller neodymium "disk" to the directly to the top of each polepiece instead. They stuck really strong. Left them on a few days.
I thought it would be fun to hear what my bass sounded like with the "disks" still on. The magnetism was so strong that the strings has awkward vibration issues, but the volume was HUGE.
After experimenting with the magnets, I found that my pickups were more balanced and pronounced. I just wonder whether the re-magnetizing works better when the pickups are energized...